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Electric Spacecraft Propulsion

Why Use Electric Propulsion?

Development of electrical propulsion systems has been stimulated by limitations in conventional chemical propulsion that derive from Newton's laws of dynamics. A rocket-propelled spacecraft derives its acceleration from the discharge of propellant and its equation of motion (neglecting gravitation and drag) follows directly from the conservation of the total momentum of the vehicle and the propellant stream:

Time varying (instantaneous) mass of the vehicle

Vehicle acceleration

Velocity of the exhaust stream

Rate of change of spacecraft mass due to propellant expulsion

The product of the rate of mass expulsion and the exhaust velocity is the thrust generated by the propulsion system:

Thrust generated by propulsion system

This thrust can be treated, for most purposes, as if it were an external force applied to the vehicle.

The integral of the thrust over the time for which it is applied is the impulse, or change of momentum. The ratio of thrust to the rate of expulsion of propellant measured in units of weight expelled per second is known as the specific impulse:

Specific impulse

Gravitational acceleration at sea level

Rate of expulsion of propellant measured in units of weight per second

If the exhaust velocity is constant during the thrusting time, the spacecraft experiences an increment in velocity which is linearly dependent on the exhaust velocity and logarithmically dependent on the propellant mass ejected:

Increment in velocity

Velocity of the exhaust stream

Vehicle mass at beginning of thrust period

Vehicle mass at end of thrust period

The deliverable mass fraction, the proportion of the spacecraft's initial mass that can be delivered to the vehicle's destination, is a negative exponential in the scalar ratio of the required velocity increment to the exhaust velocity:

The velocity increment required for a given mission or manoeuvre is an indication of the energetic difficulty of that mission or operation. If a spacecraft is to deliver a large proportion of its initial mass to its destination, the deliverable mass fraction equation shows that the propulsion system exhaust velocity must be comparable to the required velocity increment.

Chemical spacecraft propulsion systems create thrust by thermodynamically expanding heated propellant gas through a nozzle. The energy to heat the propellant is stored in the chemical bonds of the propellant or propellant / oxidiser combination and released through decomposition in single propellant systems or chemical reaction in multi-propellant systems. Chemical propulsion systems are limited by the available reaction energies and thermal transfer considerations to exhaust gas velocities of a few thousand metres per second. However, many desirable future space missions require velocity increments that are an order of magnitude, or more, higher than this.

For missions that require a high velocity increment, an alternative method of propulsion having a higher specific impulse or exhaust gas velocity than can be achieved using chemically fuelled thermodynamic expansion is required. Electric spacecraft propulsion offers just this possibility.

Electric spacecraft propulsion systems create thrust by using electric, and possibly magnetic, processes to accelerate a propellant. More intense forms of propellant heating, as used in electrothermal propulsion systems, offer one possibility for increased exhaust velocity, but encounter limitations due to restrictions on the temperatures that can be sustained by engine components in contact with the propellant gas flow. Thermodynamic expansion can be abandoned in favour of direct application of body forces to particles in the propellant stream. This is the method used by electrostatic and electromagnetic propulsion systems.

Comparison of propulsion technologies

Chemical

Electric

Small monopropellant thruster

Fregat Main Engine (S5.92M)

SMART-1 Hall Effect Thruster (PPS-1350)

Propellant

Hydrazine

Nitrogen tetroxide / Unsymmetrical dimethyl hydrazine

Xenon

Specific Impulse (s)

200

320

1640

Thrust (N)

1

1.96 x 104

6.80 x 10-2

Thrust time (s)

1.66 x 105

877

1.80 x 107

Thrust time (h)

46

0.24

5000

Propellant consumed (kg)

52

5350

80

Total Impulse (Ns)

1.1 x 105

1.72 x 107

1.2 x 106

Fregat produces ~ 14 times the total impulse of SMART-1's engine, but uses nearly 70 times more propellant mass to do so. The hydrazine thruster produces less than a tenth as much total impulse while using 65% of the propellant mass.

Geostationary communications satellites have used electric propulsion systems for station keeping since the early nineteen-eighties. Low Earth orbit satellites, such as the Iridium mobile communications cluster, have also used electric propulsion for orbit adjustments but the use of electric propulsion as a spacecraft's primary means of propulsion has been restricted to experimental vehicles such as NASA's Deep Space One, which was equipped with a xenon ion engine.

ESA's SMART-1 spacecraft is equipped with an electric propulsion system as its primary means of propulsion. It is intended as a technology demonstrator for the use of primary electric propulsion on future missions.